A single photon emission system is described in the International Patent Application WO 2009/105814 A1. A pump source generates optical pump radiation which is directed to a single photon emitter by a microscope objective. The optical pump radiation optically excites the single photon emitter to emit single photons which are transmitted to the microscope objective. The propagation direction of the single photons is opposite to the propagation direction of the optical pump radiation. A beam splitter is used to separate the back-travelling single photons from the optical pump radiation.